Thermal cracking process with an improved



J. w. DAvsoN THERMAL CRACKING PROCESS WITH AN IMPROVED HEAT CONSERVATIVEQUENCHING TECHNIQUE Filed Dec. 19, 1955 Aug. 11, 1959 mwmmom l L wm mmmommumnzzonfi tates United Joseph W. Davison, Bartlesvilie, Okia.,assign'or to Philiips Petroleum Company, a corporation of DelawareApplication December 19, '1955, Serial No. 553,'754

4 Claims. (Cl. 260-683) This invention relates to a method of andapparatus for forming low molecular weight gases by thermally crackinghydrocarbons.

It is known that ethylene and other low molecular Weight unsaturatedhydrocarbons can advantageously be produced by the thermal cracking oflight hydrocarbons such as ethane, propane or butane. The presentinvention is concerned primarily with the problem of quenching theeflluent gases of such a process in a manner so as to obtain a maximumyield of desred products with a minimum of heat exchanger surface andcooling tower requirements. It is also important to avoid the depositionof coke and tars in the transfer conduits, heat exchangers and otherpieces of equipment. It is known in the art to quench the productsresulting from the pyrolysis of hydrocarbons by means of fluids such aswater and oil. However, the quenching systems employed heretofore haveseveral disadvantages. If Water alone is used as the quench medium, theinvestment in heat exchangers and cooling towers is excessive and, whilecoking at the primary quench point is avoided, the quenched productstend to carry finely dispersed tars into equipment located downstreamfrom the primary quench. Furthermore, recovery of Waste heat cannot beaccomplished eifectively when using a water quench alone, The use of oilas the quench medium has the advantage of removing tars from the gasstream. However, oil alone is less satisfactory as the primary quenchbecause of coking in the transfer conduits and the relatively largeVolume of oil required.

The present invention employs a combination of water and oil quenchirgsteps. Also, novel heat exchange steps are integrated with the gasseparation system in such a manner as to avoid the difliculties of thesystems pre- Viously known while accomplishing a maximum heat recoverywith a minimum amount of equipment. The invention comprises, generally,vaporizing a hydrocarbon feed stream and directing the vapors into acrackng furnace. The efiluent from the cracking furnace is quenched bywater and passed to a quench drum where it is contacted by a cool oil.The eflluent from the quench drum is cooled, compressed and passed to anabsorber column. The hot oil from the quench drum is passed in heatexchange relationship with the liquid in the lower portion of theabsorber. This cools the oil and Supplies heat to the absorber. The oilis then passed in heat exchange relationship with the incoming feedstream to provide the initial vaporizaton thereof. The oil can then becooled further, if desired, and is recirculated to the quench drum.

Accordingly, it is an object of this invention to provide an improvedprocess and apparatus for producing low molecular weight gases bythermally cracking hydroca-rbons.

Another object is'to provide an improved quench system for use withpyrolysis systems.

A further object is to provide improved heat exchange 2,39%,475 PatentedAug. 11, 1959 ice 2 means for hydrocarhon cracking and gas separatingsystems.

Other Objects, advantages and features of the invention should becomeapparent from the following detailed description taken in -conjunctionwith the accompanying drawing Which is a schematic representation of apresent preferred embodiment of the invention.

Referring now to the drawing in detail, the feedstream' to be cracked issupplied by an inlet conduit 10 Which has a pump 11 therein. Conduit 10communicates With a heat exchanger 12 which provides for thevaporization of the feedstream. The outlet of conduit 10 communicateswith the inlet of a cracking furnace 13. A pressure recorder-controller14 and a flow recorder-controller 15 are associated with conduit 10 tosupply the feed stream to furnace 13 at a constant pressure and aconstant rate. A diluent, such as steam, is introduced into conduit 10through a conduit 17 which has a flow recorder-controller 18 associatedtherewith.

Funnace 13 has an elongated pipe 20 in the upper portion thereof whichcommunicates with a second elongated pipe 2.1 in the lower portion. Heatis supplied to these pipes by a plurality of burners, not shown, whichare mounted in the side Walls of the furnace. Pipe 21 communicates 'withan outlet conduit 22. Quench water is introduced into conduit 22 througha conduit 23 at a constant rate which is maintained by a flowrecordercontroller 24. The setting of controller 24 is established by atemperature recorder-controller 26 which responds to the temperature inconduit 22 downstream from conduit 23. The outlet of conduit 2 2communicates with the lower in'letcf a quench drum 27. A quench oil isintroduced into an upper inlet of drum 27 from a conduit 23. A constantflow of oil through conduit 28 is maintained by a flowrecorder-controller 29 which is associated with the conduit. ThegaseSfrom furnace 13 pass upwardly through drum 27 in countercurrent flowwith the descending quench oil. These gases are removed from drum 27through a conduit 30 Which communicates through a cooler 31 with aknock-out drum 33. Any liquid which is condensed in drum 33 is removedthrough an outlet conduit 34. The gases in drum 33 are passed through aconduit 35 to the inlet of a compressor 36. The outlet of compressor 36is connected by a conduit 37 to the inlet of an absorber column 38.

A lean absorbing oil is introduced into the upper portion of column 38through a conduit 39. Gases are removed from column 38 through a conduit40 and are directed to additional separating apparatus, not shown. Therich oil is removed from absorber 38 through a conduit 42 and passedinto a stripper, not shown. A portion of the rich oil is directed by aconduit 43 through a heat :exchanger 44 and is returned to the lowerportion of absorber 38. Heat is supplied to exchanger 44 by passing aheat exchange medium therethrough. A portion of the liquid in the lowerregion of absorber 38 is passed by a conduit 45 having a pump 46 thereinthrough a heat exchanger 47 and is returned to the column. The hotquench oil from drum 27 is passed through heat exchanger %7 by a conduit50 which has a pump 51 and a coke filter 52 therein. This oil is passedthrough conduit 50 at a rate which is maintained by a liquid levelcontroller 53 that responds to the liquid level in drum 27.

The quench oil removed from heat exchanger 47 is passed through aconduit 55 to heat exchanger 12. Additional heat is removed from thequench oil in exchanger 12 and this vaporizes the feedstream deliveredto furnace 13. The cooled oil from heat exchanger 12 is passed through aconduit 56 to a storage tank 57. Conduit 56 has a valve 57 therein whichis adjusted by atemperature indicator-controller 58 that responds to thetemperature of the oil downstream from valve 57. A by-pass conduit 59directs a portion of the oil through a cooler 60 instead of throughvalve 58. The position of valve 58 thus determines the ratio of oilpassed through cooler 60 to the oil passed directly through Valve 58. Inthis manner the oil is delivered to storage tank 57 is cooled to apredetermined temperature. Obviously, the heat of the oil may be used toheat other fiuid streams in some applications of this invention, therebyavoiding the 'loss of heat at cooler 60. The oil is subsequently removedfrom tank 57 and passed by a pump 61 back through conduit 38 to quenchdrum 27. A conduit 62 is provided to remove sludge from tank 57.

As a specific example of the operation of the thermal cracking processof this invention, a mixture comprising normal butane and isobutane isintroduced into furnace 13 through conduit '10. These two gases aresupplied in the ratio of 972 mols of normal butane to 28 mols ofisobutane. For each one thousand mols of feed gas, 249 mols of water inthe form of steam are added through conduit 17. The resulting mixture isintroduced into furnace 13 at a temperature of approximately 150 F. andat a pressure of approximately 85 pounds per square inch absolute(p.s.i.a.). The residence time of the mixture in pipe 20 isapproximately 7 seconds and the residence time in pipe 21 isapproximately 4 seconds. The gases are heated in pipe 20 to atemperature of approximately 1000 F. and are further heated in pipe 21to a temperature of approximately 1500 F. The eflluent gases leavefurnace 13 at a temperature of approximately l500 F. and at a pressureof approximately 20 p.s.i.a. The effluent from fumace 13 has acompostion approximately as follows.

These gases are quenched to a temperature of approximately 900 F. by thedirect introduction of water into conduit 22. This can be accomplishedby introducing 1240 mols of water at a temperature of approximately 100F. for each 1000 mols of feed gases.

Quench oil is supplied to drum 27 at a temperature of approximately l40F. and at the rate of 1920 moles per 1000 moles of feed gas. Gases areremoved from drum 27 at a temperature of approximately 180 F. and arecooled to 100 F. by heat exchange with water in exchanger 31. The gasesare compressed in a several stage compressor having intermediate coolingand finally enter absorber 38 at a temperature of 100 F. and at apressure of 170 p.s.i.a.

Quench oil is removed from drum 27 at a temperature of 375 F. and passedthrough reboiler 47. The oil leaves reboiler 47 at a temperature of 315F. and is passed to feed vaporizer 12. The oil removed from vaporizer 12is at a temperature of 250 F. and is cooled by cooler 60 to atemperature of approximately 140 F.

It should be noted that the temperature to which the efiluent gases arecooled in drum 27 is above the dew point so that water introduced in theprimary quench is not condensed in drum 27 This eliminates the need foran oil-water separator. However, the gas is cooled initially so that theamount of additional oil cooling required is minimized. The step ofquenching the furnace effluent to 900 F. by the water quench minimizesthe amount of 4 equipment required, stops the pyrolysis reaction beforevaluable products are destroyed by side reactions and prevents thedeposition of tars and coke in the transfer conduits. The furtherquenching from 900 F. to 180 F. by the oil results in effectivescrubbing of coke and tars from the gas and makes possible substantialheat recovery by heat exchange in the absorber reboiler and the butanevaporizer.

` The quench oil employed can be any heavy oil, paraf-` fin or aromatic,which will flow and can be sprayed at the coolest temperatures involved.A present preferred oil is a heavy aromatic oil Which is produced in thecracking operation and is permitted to accumulate. The following dataare characteristic of such an oil:

Specific gravity at 60/ 60 F.=l.0655

Kinematic viscosity at F.=l4.38 centistokes Kinematic viscosity at 210F.=2.86 centistokes Bureau of Mines Correlation Index=133 ASTMDistillation, corrected to 760 mm.

While the invention has been described in conjunction With a presentpreferred embodiment, it should be evident that are designatedcompositions, temperatures and pressures are merely illustrative of theoperation of the invention and that the invention is not limitedthereto. In addition to butane, the feedstream can comprise ethane,propane, refinery gases or mixtures of these various materials. Ingeneral, butane is more economical than propane because greater amountsof valuable olefin and aromatic by-products are produced from thecracking of butane. Obviously, these temperatures and pressures can bevaried over considerable ranges depending upon the gases to be cracked,the desired products and various economic factors.

What is claimed is:

1. A process for producing low molecular weight gases which comprisesheating a hydrocarbon feed stream to vaporize at least a portion of thestream, further heating the stream to effect thermal crackng of asubstantial portion of the stream, adding water to the cracked stream toeffect cooling, then directly passing the cooled cracked stream incontact with a cooled quench oil and thereby scrubbing tars from saidcracked stream and further cooling said cracked stream, passing theresulting cooled cracked stream to a heated absorbing zone, contactingthe cooled cracked stream in the absorbing zone with a lean absorbingmedium, passing the resulting heated quench oil in heat exchangerelationship with fluid in the absorbing zone to heat the absorbingzone, thereafter passing the quench oil in heat exchange relationshipwith the hydrocarbon feed stream to effect the first-mentioned heatingthereof, and recycling the quench oil to contact additional crackedproducts.

2. A process for producing ethylene which comprises heating ahydrocarbon feed stream to vaporize at least a portion of the stream,further heating the stream to effect thermal cracking of a substantialportion of the stream, to produce a substantial quantity of ethylene,adding water to the cracked stream to cool same to about 900 F., thendirectly passing the resulting cooled cracked stream in contact with acooled hydrocarbon quench oil and thereby scrubbing tars from saidcracked stream while also cooling said stream to about 180 F., saidtemperature being above the dew point so that water is not condensedpassing the resulting cooled cracked stream to a heated absorbing zone,contacting the cooled cracked stream in the absorbing zone with a leanabsorbing medium, passing the resulting heated quench oil in heatexchange relationship with fluid in the absorbing zone to heat theabsorbing zone, thereafter passing the quench oil in heat exchangerelationship with the hydrocarbon feed stream to effect thefirst-mentioned heating thereof, and recycling the quench oil to contactadditional cracked products.

3. The process in accordance With claim 2 Wherein the feed streamcomprises paraflins having four carbon atoms per molecule, and Whereinsaid thermal cracking is provided by heating the feed stream to atemperature of about 1500 F. in a period of about 11 seconds.

4. A process for producing low molecular Weight gases Which comprisesbeating a hydrocarbon feed stream to vapon'ze at least a portion of thestream, further heating the stream to efiect thermal crackng of asubstantial portion of the stream, adding Water to the cracked stream toeffect cooling, then directly passing the cooled cracked stream incontact with a quench oil and thereby scrubbing tars from said crackedstream and further cooling said cracked stream, said contact beingeftected under conditions such that water is not condensed andseparating the resulting water-free quench oil stream, passing theresulting cooled cracked stream to a heated absorbing zone, contactingthe cooled cracked stream in the absorbing zone with a lean absorbingmedium, passing the hot water-free quench oil in heat exchangerelationship With fluid in the absorbing zone to heat the absorbingzone, thereafter passing the quench oil in heat exchange relationshipwith the hydrocarbon feed stream to effect the first-mentioned heatingthereof and recycling the quench oil to contact additional erackedproducts.

References Cited in the file of this patent UNITED STATES PATENTS2,363,903 Smith Nov. 28, 1944 2,414,817 Kleiber et al Jan. 28, 19472,498,806 Hachmuth Feb. 28, 1950 2,503,202 Johnson et al. Apr. 4, 19502,621,2.16 White Dec. 9, 1952 2,712,538 Wadsworth July 5, 1955 2,780,580Kniel Feb. 5, 1957

2. A PROCESS FOR PRODUCING ETHYLENE WHICH COMPRISES HEATING AHYDROCARBON FEED STREAM TO VAPORIZE AT LEAST A PORTION OF THE STREAM,FURTHER HEATING THE STREAM TO EFFECT THERMAL CRACKING OF A SUBSTANTIALPORTION OF THE STREAM, TO PRODUCE A SUBSTANTIAL QUANTITY OF ETHYLENE,ADDING WATER TO THE CRACKED STREAM TO COOL SAME TO ABOUT 900* F., THENDIRECTLY PASSING THE RESULTING COOLED CRACKED STREAM IN CONTACT WITH ACOOLED HYDROCARBON QUENCH OIL AND THEREBY SCRUBBING TARS FROM SAIDCRACKES STREAM WHILE ALSO COOLING SAID STREAM TO ABOUT 180* F., SAIDTEMPERATURE BEING ABOVE THE DEW POINT SO THAT WATER IS NOT CONDENSEDPASSING THE RESULTING COOLED CRACKED STREAM TO A HEATED ABSORBING ZONE,CONTACTING THE COOLED CRACKED STREAM IN THE ABSORBING ZONE WITH A LEANABSORBING